It is known in the art that a specific type of optical interferometer known as a fiber optic gyro (FOG) uses the theory of relative motion to calculate the angular velocity or rotation rate of a body. A FOG typically consists of a light source, an optical loop, a beam splitter and combiner, a phase modulator, and an optical detector. Typically a light wave is injected into both ends of a single fiber optic cable shaped in a circle (called a ring or loop). The beam splitter serves to split the light wave from the light source into two substantially equal waves and sends them propagating in opposite directions around the optical loop. The beam combiner serves to combine the waves after they have traveled around the loop and the optical detector measures the intensity of the combined wave. The beam splitter and combiner is typically one component. One or more phase modulators, placed in one or both ends of the loop, may be used to induce a phase shift between the waves.
When the optical ring is at rest, i.e., not subject to rotation, the two counter propagating light waves, combined by the beam combiner and monitored by the optical detector, will be exactly in phase because the distance traveled by both waves is exactly the same. When the waves are in phase, they combine for a maximum intensity at the optical detector. When the ring is rotated about an axis normal to the plane of the ring, the wave traveling in the direction of rotation will require more time to reach the end of the fiber (where the waves are combined), than the wave traveling in the opposite direction. This occurs because the detector is moving away from one wave and toward the other. Therefore, the length of travel is shorter for one wave than the other. This path difference causes a nonreciprocal (differential) phase shift between the two waves such that when the waves are combined, the optical intensity is not a maximum. The phase shift induced by the angular rotation rate of the ring is known as the "Sagnac" effect.
For the purpose of closed loop operation, as is known in the art, such phase shift can be compensated for, i.e., nulled, by imposing a phase shift on the waves equal and opposite to the rotation induced (Sagnac) phase shift such that the phase difference between the waves is brought to zero, restoring the maximum intensity at the optical detector. The phase shift imposed to produce the desired null in closed loop operation serves as a measure of rotation rate and provides the same starting intensity for each rotation rate thereby providing consistent sensitivity to rotation rate measurement. There are various techniques for imposing this phase shift to null the Sagnac phase shift, one of which is a periodic ramp (also known as a sawtooth waveform, or serrodyne waveform) applied to parallel electrode plates located around optical waveguides at one or both ends of the optical loop. A voltage applied to the plates induces a proportional phase shift in the waves propagating between the plates. By controlling either the amplitude or the frequency of the waveform one can inject various phase shifts at different times on the waves. For example, in a fixed amplitude serrodyne modulation closed loop FOG, the change in serrodyne ramp frequency needed to null the Sagnac phase is proportional to the rotation rate of the ring.
In addition to using a serrodyne waveform to counteract the rotationally induced Sagnac phase difference, a technique is typically employed to increase the sensitivity of the closed loop system to changes in rotation. One such technique utilizes the property that the combined wave intensity behaves like a vertically shifted (raised) cosine curve (i.e., a curve with a non-zero average value having the negative-most point at zero), where the intensity seen at the optical detector is related, by this curve, to the phase difference induced by rotation. When the ring is at rest, the combined wave intensity is at a maximum and thus the intensity is at the peak of the cosine curve. However, at the peak, the sensitivity is at a minimum because the slope is effectively zero at this point. Therefore, any change in phase shift will produce a minimal change in intensity, yielding minimal sensitivity. What is commonly done in the art to increase sensitivity is to shift the operating point, or the non-rotation null, from the maximum of the cosine curve to a region where the slope is much steeper, i.e., at either .pi./2 or -.pi./2. This can be accomplished by applying a constant or DC phase shift (or DC sensitizing source). However, if a DC phase shift is induced at .pi./2, the steep slope of the cosine curve at .pi./2 creates a high sensitivity to phase changes requiring a very stable and accurate DC source to avoid erroneous rotation rate readings. It can be difficult to achieve a very constant DC phase shift over time, and any change in this DC shift will induce an incorrect indication of rotation rate. Instead, systems will typically modulate or change the operating point in time from one side of the cosine curve where the intensity is most sensitive, to the other side of the cosine curve where it is also most sensitive. This modulation is hereinafter referred to as a sensitizing oscillation (i.e., AC sensitizing). This is achieved by driving the modulator with a waveform such that the phase shift between the counter-propagating waves varies in a known way. If such a modulation source is used to improve sensitivity, a demodulator is needed at the output of the optical detector to detect signal components generated by rotation. Typically what is used is a synchronous demodulator driven at the same frequency as the sensitizing oscillator source (also known as the fundamental frequency). This is also referred to in the literature as a lock-in amplifier. When a phase shift occurs, due to rotation, components at the output of the demodulator change in a predictable way allowing the calculation of rotation rate.
A typical closed loop fiber optic gyro will contain both a sensitizing oscillator and a synchronous demodulator, as well as a sawtooth waveform driver placed in a closed loop configuration. The sensitizing oscillator and synchronous demodulator are provided for improved sensitivity and the sawtooth waveform generator is provided to close the loop, e.g., to null any phase differences due to rotation rates. Therefore, in a closed loop serrodyne waveform FOG, both the sensitizing oscillator and the sawtooth waveform are simultaneously applied to the optical phase modulator. For a given rotation rate there will exist a corresponding frequency and amplitude of the sawtooth waveform that will null the induced phase shift.
Although it is possible to drive the optical phase modulator with a "zero" frequency sawtooth waveform when the ring is at rest, it is known that using a non-zero frequency may be more practical. When at rest there are numerous ramp frequencies, including zero, which will null the gyro output. If the frequency selected is zero, the non-rotating phase shift will remain at null independent of variations in certain physical parameters of the FOG. However, using "zero" frequency may be impractical because it requires a very accurate reversal of ramp polarity when the rotation direction reverses. If this accuracy is not achieved, large errors may result. An alternative to using "zero" frequency is to use one of the non-zero output nulling frequencies when the gyro is at rest. However, the allowable non-zero output nulling frequencies may drift as physical parameters of the FOG change. This drift will create a false indication of phase shift causing an erroneous rotation rate indication thereby degrading the accuracy in measuring FOG rotation rate.
More specifically, it is known that a FOG produces accurate rotation measurements when the zero rotation output nulling frequency is set to some integer multiple of the loop eigenfrequency, Fe. This frequency is defined as half the reciprocal of the loop delay time T and is related to physical parameters of the system by the following relationship: EQU Fe=1/2T=C/2nL (1)
where C is the speed of light in a vacuum; n is the index of refraction for the loop optical medium; and L is the length of the loop optical fiber or waveguide. Equation 1 shows that the loop eigenfrequency (Fe) will change with changes in either n or L. For example, a change in temperature will cause a change in the length L of the loop. Additionally, a change in temperature may cause a change in the optical properties of the fiber and therefore, a change in the fiber refractive index. These changes in the fiber length and refractive index change the loop transit time, and therefore, the loop eigenfrequency (Fe).
In the closed loop fixed amplitude serrodyne drive FOG, when the ring is at rest, the drive frequency (Fd) is driven to the loop eigenfrequency (Fe) or a multiple thereof, n.Fe, to achieve the nonrotation intensity setpoint (null point). When the ring is subjected to a rotation rate, closed loop control logic will force the drive frequency to a new value to achieve the nonrotation phase shift. This change in drive frequency is proportional to rotation rate. This technique is accurate provided the Fe in the control logic (with respect to which the change is measured) matches the loop Fe of the FOG. However, if the physical characteristics of the FOG change, causing the loop Fe to change, there will be a corresponding intensity change that the control logic will compensate for by changing the drive frequency to maintain the nonrotation intensity. This intensity change is induced by the periodic modulation drive waveform acting on the counter-propagating waves whose propagation time has been changed by the change in the physical characteristics. Because the control logic is unaware of the loop Fe shift, this change in drive frequency will manifest itself as an erroneous rotational reading.
It is also known in the art that, to achieve optimal performance, the fundamental frequency for the sensitizing oscillator and the synchronous demodulator can be set at Fe. However, if the loop Fe drifts, this optimal performance will be compromised.